This is a minimal example to show how you can render arbitrary Rmd files on a website based on blogdown. The key is that you add an R script build.R under the R/ directory (in which you can use one line of code blogdown::build_dir('static')), and put your Rmd files under the static/ directory. See the brief documentation in the blogdown book.
It does not matter which output formats your Rmd files are generated to; build_dir() will call rmarkdown::render() to render them. On this example website, I provided two examples (R packages xaringan and bookdown are required):
Generating an HTML5 presentation based on xaringan: slides/xaringan.html (Rmd source: static/slides/xaringan.Rmd).
Generating a PDF document using the bookdown::pdf_document2 format: pdf/example.pdf (Rmd source: static/pdf/example.Rmd).
Below is a list of example blog posts on this website rendered by Hugo instead of rmarkdown: